Echolocation, the main sensory modality of odontocetes and microbats, has mostly been studied using single receivers. Such a recording setup is sufficient to study signal parameters like pulse interval, inter click interval or terminal frequency. However, the usually high frequencies and high directionality of echolocation signals do not allow precise measurements of start frequency, signal intensity and emission directionality using a single receiver. Recording impinging signals with multiple receivers in a defined spatial arrangement allows sampling the emitted signal simultaneously at different angles relative to the acoustic axis. Such arrays have first been used to study emission directionality of stationary animals at a known location relative to the array. Starting in 1990, arrays were used more extensively to first locate an echolocator based on the time of arrival difference of the emitted call at the different receivers. In a second step, the signal parameters of the recorded sounds were then analyzed and measurements of first signal intensity and later directionality of free ranging animals were obtained.
In the course of this doctorate, I first used a two dimensional 16 microphone array to measure the signal intensity and the variation thereof of big brown bats in a flight room. The position of the animals was determined by stereo infrared video recordings. I confirmed that the signal intensity decreased as the bats flew across the flight room towards a landing platform. The intensity reduction however was not constant but showed oscillations of up to 12 dB within a few calls. These oscillations were linked to the wingbeat, indicating an effect of wing movement on the call intensity. In addition, the call emission timing was linked to the wingbeat cycle. Detailed analysis revealed that single calls were emitted during the upstroke. When the bats emitted groups of calls, signal emission started earlier during the wingbeat cycle.
The two dimensional microphone array was then increased in size to four by four meters and adapted for recordings of free ranging bats in the field. Bats were first localized by videogrammetry, later during the doctorate by acoustic localization. Introducing the latter to the department of animal physiology was one of my main contributions during my doctorate. I developed additional algorithms which resulted in the ability to separate signals emitted by multiple individual echolocators and to measure the sonar parameters of each individual.
In addition to bats, I studied the directionality and source levels of porpoise clicks. In a first approach, I used a two dimensional plus shaped 16 hydrophone array to measure the directionality of single clicks emitted by a stationary animal in both the horizontal and vertical dimension separately. The sonar beam was found to be narrower than reported previously and the directionality to some extend dynamic. To exclude individual differences of the two animals used in the two studies, I carried out a further experiment. Clicks by two additional porpoises were recorded with a regularly spaced wall of 15 hydrophones while catching prey. The porpoise’s position at click emission was determined using acoustic localization and the intensity, directionality and direction of signal emission relative to the body movement were studied. The results from these three freely moving animals confirmed the narrow beam in harbor porpoises.